Scientists
have sequenced the genome of a bacterium that lives inside the gut of
the blood-sucking tsetse fly. The fly transmits a parasite causing the
deadly African sleeping sickness. The bacterium and fly live in symbiosis,
and scientists hope that studying the bacterium's genome will help in
developing better controls for the spread of the disease.

Tsetse fly. Courtesy Serap Aksoy

Wigglesworthia glossinidia is on par with some of the smallest
genomes sequenced. The bacterium has co-evolved with its insect host over
millions of years; this co-evolution has allowed the bacterium to streamline
its genome, eliminating genes found in its host.

"The Wigglesworthia genome has shrunk," says Serap Aksoy
of Yale University School of Medicine in New Haven, Connecticut, who led
the sequencing project and first named the organism. Wigglesworthia
bears the name of British entomologist Sir Vincent Brian Wigglesworth,
who described the organism beautifully, says Aksoy.

Researchers compared Wigglesworthia to the well-studied intracellular
Buchnera bacterium, which lives symbiotically in aphids. Although
there are similarities between the two bacteria, the genomes are actually
quite different.

Unlike Buchnera, Wigglesworthia's genome still contains
remnants of a free-living organism, such as genes for motility. Although
scientists have never seen Wigglesworthia swim, they found genes
that synthesize flagellawhip-like cellular propellers.

The flagella may help Wigglesworthia travel from adult tsetse
flies to larvae, says Aksoy. The female tsetse fly fertilizes its young
in its uterus, and the bacteria are then transferred through the mother's
milk. One theory for flagella is that they may aid bacteria in locomotion
to or invasion of the larval cells.

The Wigglesworthia genome also contains over 60 genes involved
in the synthesis of vitaminsnutrients that the tsetse fly relies
on for its fertility. This finding confirms previous studies that have
indicated that the fly depends on the bacteria to provide these vitamins
not found in its restricted diet of blood. Without the bacteria (and vitamins),
the tsetse fly is sterile.

This sterility may someday be used to prevent the spread of disease.
By removing the bacteria from tsetse flies, scientists would stop the
development of offspringreducing fly populations and disease transmission.
Field studies are already underway testing the efficacy of sterility in
tsetse flies.

The tsetse fly is the carrier of African trypanosomes, the parasite that
causes the deadly sleeping sickness. Aksoy says she plans to investigate
the "big picture" interactions between the parasite, fly and
symbiotic bacteriaincluding whether the bacteria supply essential
nutrients not only to the tsetse fly, but also to the parasite.

"Wigglesworthia has a fascinating biology," says Aksoy,
"but stopping transmission of disease is always the most important
background to our work."

Aksoy maintains one of only a handful of tsetse fly colonies in the world.
She breeds five different species of tsetse flies, with about 5,000 breeding
adults at any one time. Members of her lab feed the hungry mouths a diet
of bovine blood, which the flies suck up through a specialized membrane,
not from live animals.

The research team does not work with flies that have parasites that infect
humans, but rather a sibling species that causes disease in animals. The
fly-holding cages are "well-designed" so bites are "a rare
event" in the laboratory. Nevertheless, lab coats are a must, and
the scientists leave their smock in the insectary at the end of each day.